Communications cables which are strung between poles or those which are buried in the ground are subjected to abuse such as, for example, attack by animals, mechanical abrasion and crushing. Attacks by gophers on buried cable and by squirrels on aerial cable have been a continuing concern. Gophers, for example, have been shown to exert biting pressures as high as 124,044 k Pa. Cables having an outside diameter below a size of about 2 cm in diameter are more apt to be damaged than larger cables because the animals can bite directly down on them. For larger size cables, only a scraping or raking action takes place. In fact, on cables exceeding about 5.1 cm in diameter, gopher attack is rarely observed.
An excellent discussion of this problem was presented at the 25th International Wire and Cable Symposium. A written version of that presentation appears beginning at page 117 in the proceedings of that conference being authored by N. J. Cogelia, G. R. LaVoie, and J. G. Glahn and being entitled "Rodent Biting Pressure and Chewing Action and Their Effects on Wire and Cable Sheath".
Many rodents, and in particular the plains pocket gopher (Geomys bursarius) and the fox squirrel (Scirus niger), habitually chew on communication cables. Such chewing frequently causes immediate damage to the cable's wiring or optical fibers or results in damage when rain water or other moisture enters holes gnawed through protective outer jacketing. In the case of rodents, chewing on objects which are tough in composition is necessary to prevent their ever-growing incisor teeth from overgrowing. Thus, for relatively small cables, simply providing a hard outer cable jacket is insufficient and may even provide an inducement rather than a deterrent to chewing by rodents.
Because it is frequently undesirable simply to kill the rodents, it is necessary to devise non-lethal apparatus and methods whereby rodents will be prevented or deterred from attacking cables. Numerous psychological studies have been performed which show that psychological barriers can be induced by causing an undesirable effect to result immediately after an act by an animal is performed. The animal then associates the undesirable result with the immediate preceding act and thereafter changes its behavior to prevent the undesirable result. In one prior art patent, this knowledge is utilized to incorporate, in a cable assembly, a non-lethal but toxic chemical agent which is capable of causing rapid illness thereby inducing a psychological barrier resulting in a behavioral change opposed to the cable chewing.
It has been found that an effective way to protect directly exposed cables from rodent attack is to wrap them in a metallic shield. A longitudinally applied shield, if otherwise suitable, would be preferable from a manufacturing economy standpoint. The prior art includes the use of stainless steel shields to protect against rodent attacks. Stainless steel is used so that after an initial attack, partial destruction of the integrity of the shield does not lead to corrosion and subsequent holes caused by exposure to moisture. For cables above the critical size, the use of a corrugated shield having a longitudinally overlapped seam generally has provided sufficient protection. However, in the smaller sizes, inadequate shield arrangements can lead to failures. Rodents have been able to encompass the cable with their teeth and pull open the seam.
In another prior art approach to rodent protection, an all-dielectric optical fiber cable is disposed within an extruded plastic duct having an inner diameter which is significantly larger than the outer diameter of the cable. Such a structure is disclosed in an article entitled "Fiber Cable Wears An Extruded jacket" which was authored by G. J. Beveridge, et al., and which appeared in the Apr. 15, 1985 issue of Telephone Engineer and Management beginning at page 100. The outer diameter of the duct is sufficiently large to prevent rodents from enveloping the duct with their jaws.
The hereinabove described solutions have not been totally satisfactory. The use and handling of toxic chemicals is certainly not desired. Further the additional expense in the material cost and the application of a stainless steel shield or of an extra duct are solutions which certainly invite thought to provide alternatives.
The sought-after cable must be cost-effective and easily manufactured. Hopefully, it provides the cable with protection against rodent attacks without compromising other properties of the cable such as, for example, its flexibility. Also, the sought-after protection must be such that it can be included as a portion of the sheath system for use in any cable be it one which includes optical fibers or metallic conductors.
There is also a desire that the sought-after cable include an all-dielectric sheath system. Many cables are installed in areas which are characterized as medium to high lightning areas. Both buried and aerial cables are damaged by lightning strikes.
Metallic sheathed fiber optic cables can be affected by lightning in several ways. Thermal damage, that is burning, charring and melting of the metallic sheath components, is caused by the heating effects of the lightning arc and a current being carried to ground by the metallic members of the core or sheath. In buried cables, a second mode of damage is mechanical, causing crushing and distortion of the sheath. This results from an explosive impact sometimes called a steamhammer effect, which is caused by the instantaneous vaporization of water in the earth in a lightning channel to the cable. A direct lightning hit may very well destroy the portion of the cable at the location of the direct hit. Indirect hits may cause pinholes which will allow water to enter the cable.
The prior art abounds with patents relating to metallic sheath systems for copper core cables such as one comprising an aluminum shield enclosed by a carbon steel shield with each having a longitudinal seam. This sheath system offers protection from mechanical damage, electromagnetic interference and lightning and its cost is quite low because it is made in a single pass at relatively high line speeds. However, the use of a shield which is made of carbon steel occasionally has resulted in long term failures, even in cables larger than 2 cm. Failure may occur because the underlying steel shield may become exposed when rodents violate the jacket. Once exposed, the steel shield, which withstands the initial attack by rodents, corrodes readily. This renders it ineffective for general mechanical protection and for protection from any subsequent rodent attack. In this regard, it should be pointed out that gophers are territorial animals which repeatedly return to areas previously occupied by them. Therefore, it is not uncommon to experience secondary attacks in the same location along a cable.
As may well be imagined, a cable having an all-dielectric sheath system which provides protection against rodent attacks as well as lightning would be welcomed. Such a cable would provide other advantages. For example, the oxidation of metallic sheaths in cable in which seawater reacts with conducting metals leads to the generation of hydrogen which could cause optical fiber transmission losses. An all-dielectric cable sheath system avoids this problem. Also, an all-dielectric cable sheath system offers survivability to electromagnetic pulse (EMP) effects.
Seemingly, the prior art does not offer a cable which is free of the above described shortcomings of presently available cables. There are cables having all-dielectric sheath system and there are cables which provide protection against rodent attacks and lightning, but there appears to be no such sheath system which provides both kinds of protection. Such a cable which has long been sought-after must be cost-competitive. Further, it must include a sheath system which is easily removed to access safely the core which could include, for example, optical fibers.